Echo cancellation circuit

ABSTRACT

A system and method for echo cancellation in digital subscriber line (DSL) service using passive devices. The outgoing transmitted signal is attenuated and fed back into the receiver circuit in such a way as to maximize the canceling of the transmitted signal and thus allow the receiver circuit to amplify and process the received signal without interference from the outgoing transmitted signal. The feedback circuit uses only passive elements. The feedback circuit has complex impedance branches. These complex impedance branches parallel the complex impedances of the transformer and transmission line such that any change in the transformer or transmission line impedance is similarly experienced in the feedback circuit. This allows for near total cancellation of the echo signal without the need for costly active circuits.

TECHNICAL FIELD

[0001] This invention relates to transmission/receiver circuits, andmore particularly to an echo cancellation circuit used in DSLcommunications.

BACKGROUND

[0002] In many communications systems a single data path transmits andreceives data signals. As an example, in digital subscriber line (DSL)service, the home user transmits and receives signals over a twistedpair of wires. At any given moment, the twisted pair of wires can becarrying both outgoing and incoming signals.

[0003] Echo cancellation circuits aid in the reception of the incomingsignals. More specifically, echo cancellation circuits compensate forthe reflection, or echo, of outgoing signals into the receiver circuit.This results in the receiver circuit receiving a cleaner incoming signalfor amplification and processing.

[0004] In general, there are two types of echo cancellation circuits.The first type includes active circuits and memories. These echocancellation circuits are trained to compensate for a particulartransmission line and terminator impedance, and to adapt to changes inthis impedance as the temperature and frequency of outgoing signalschange. The second type of echo cancellation circuit includes resistorsto reduce the reflection of the outgoing signals into the receivercircuit.

DESCRIPTION OF DRAWINGS

[0005]FIG. 1 is a circuit diagram of a resistive, passive echocancellation circuit.

[0006]FIGS. 2a, 2 b and 2 c are simplified circuit diagrams of thecircuit shown in FIG. 1.

[0007]FIGS. 3 and 4 are graphs of the relationship between impedance andfrequency.

[0008]FIG. 5 is a circuit diagram of an improved echo cancellationcircuit.

[0009] FIGS. 6-8 are simplified circuit diagrams of the circuit of FIG.5.

[0010] Like reference symbols in the various drawings indicate likeelements.

DETAILED DESCRIPTION

[0011] An improved echo cancellation circuit employs both reactiveelements, (e.g., capacitors and inductors) and resistive elements (e.g.,resistors) such that the impedance of the circuit has both real andimaginary components. This arrangement permits the circuit to moreclosely track variations in the impedance of a transmission line andassociated transformer due to variations in a frequency of a signalbeing transmitted. For ease of discussion, an echo cancellation circuitincluding resistive elements is discussed with reference to FIGS. 1-4before the improved circuit is discussed with reference to FIGS. 5-8.Referring to FIG. 1, circuit 100 includes transmitter 105 and receiver110. Transmitter 105 issues differential output signals onto nodes T and−T, while receiver 110 receives differential input signals on nodes Rand −R. A terminating resistor 115 a is coupled between nodes T and R,and a terminating resistor 115 b is coupled between nodes −T and −R. Atransformer 120 couples nodes R and −R to a twisted pair of transmissionlines 125 a and 125 b. A resistor 130 represents the impedance of thetransmitter and/or receiver circuit(s) coupled to transmission lines 125a and 125 b.

[0012] There are two cancellation circuits 135 a and 135 b forcancelling the outgoing transmitted signal. Cancellation circuit 135 ais coupled to nodes T and −R and includes three resistors 135 a 1, 135 a2 and 135 a 3. Similarly, cancellation circuit 135 b is coupled to nodes−T and R and includes three resistors 135 b 1, 135 b 2 and 135 b 3.

[0013]FIG. 2a shows only the cancellation circuit 135 a for ease ofdiscussion. It should be understood that a similar diagram and analysiscan be done for cancellation circuit 135 b. To simplify the analysis,the impedances of transformer 120 and transmission lines 125 a and 125 bare represented by impedance Z_(line). Voltage source 245 represents anincoming signal to be detected, amplified and processed by receiver 110.The outgoing signal, which is transmitted as a differential signal fromnodes T and −T, is reduced so as not to be amplified and processed as anincoming, received signal by receiver 110.

[0014] Cancellation circuit 135 a, in conjunction with receiver circuit110, operates as a voltage summer. Thus, the voltage at node A, V_(A),is given by the following equation where V_(T) is the voltage at node T,V_(−R) is the voltage at node −R, R_(ref) is the resistance value ofresistor 135 a 1, R₁ is the resistance value of resistor 135 a 2 and R₂is the resistance value of resistor 135 a 3:$V_{A} = {{V_{T}\left( \frac{R_{ref}}{R_{1}} \right)} + {{V_{- R}\left( \frac{R_{ref}}{R_{2}} \right)}.}}$

[0015] Assuming R₁=R_(ref) and R₂=½ R_(ref), this simplifies to:

V _(A) =V _(T)+(2)V _(−R).

[0016] The voltage at node −R, V_(−R) is a combination of the voltageoutput from transmitter 105, V_(−T), and the input voltage, V_(in),received through transformer 120. Thus V_(−R) may be expressed as thesum of a component, V_(−RT), provided by V_(−T) and a component,V_(−Rin), provided by V_(in):

V _(−R) =V _(−RT) +V _(−Rin).

[0017] Referring to FIG. 2b, using superposition to calculate theinfluence that V_(−T) has on V_(−R), and setting the resistance ofterminating resistor 115 b to R_(T) yields the following equation:$V_{- {RT}} = {{V_{- T}\left\lbrack \frac{Z_{line}}{R_{T} + Z_{line}} \right\rbrack}.}$

[0018] Referring to FIG. 2c and using superposition to calculate theinfluence that V_(in) has on V_(−R) when V_(−T) is grounded produces thefollowing equation:$V_{- {Rin}} = {{V_{i\quad n}\left\lbrack \frac{R_{T}}{R_{T} + Z_{line}} \right\rbrack}.}$

[0019] Substituting the equations for V_(−RT) and V_(−Rin) into theequation for V_(−R) yields:$V_{- R} = {{V_{- T}\left\lbrack \frac{Z_{line}}{R_{T} + Z_{line}} \right\rbrack} + {{V_{i\quad n}\left\lbrack \frac{R_{T}}{R_{T} + Z_{line}} \right\rbrack}.}}$

[0020] Assuming that the terminating resistor 115 b matches the combinedimpedance of transformer 120 and transmission lines 125 a and 125 b,that is Z_(line)=R_(T), the equation for V_(−R) reduces to:$V_{- R} = {{V_{- T}\left( \frac{1}{2} \right)} + {{V_{i\quad n}\left( \frac{1}{2} \right)}.}}$

[0021] Since the outgoing transmitted signal is differential, it followsthat V_(−T)=−(V_(T)) Substituting this value into the equation forV_(−R), and then substituting the equation for V_(−R) into the equationfor V_(A) yields:${V_{A} = {{V_{T}(1)} + \left\lbrack {{- {V_{T}\left( \frac{1}{2} \right)}}(2)} \right\rbrack + {{V_{i\quad n}\left( \frac{1}{2} \right)}(2)}}},$

[0022] which reduces to:

V_(A)=V_(in).

[0023] This analysis shows that the echo cancellation circuits 135 a and135 b of FIG. 1 are effective at reducing the echo of V_(T) and V_(−T)onto nodes A and B as long as the impedance of terminating resistors 115a and 115 b matches the combined impedance of transformer 120 andtransmission lines 125 a and 125 b.

[0024] As noted above, the impedance Z_(line) represents both theimpedance of the transmission lines 125 a and 125 b (e.g., the twistedpair of telephone lines outside of the user's home) and the transformer120 of FIG. 1. The individual impedances of these components vary withthe frequency of the signals they carry and the ambient temperature. Inother words, Z_(line) is not constant and does not always equal theresistances of terminating resistors 115 a and 115 b (R_(T)).

[0025] For DC signals, the impedance of transformer 120 is approximately0 Ω. Thus, the dominant factor in impedance Z_(line) is the impedance oftransmission lines 125 a and 125 b. As the frequency of the signalsincreases from 0 Hz to about 5 kHz, the impedance of transformer 120increases, which in turn causes the impedance Z_(line) to increase asshown in FIG. 3.

[0026] Above 5 kHz, the impedance of transmission lines 125 a and 125 bdecreases substantially to dominate the impedance Z_(line). Thus, forsignals above 5 kHz (e.g., from 5 kHz to 10 kHz), the impedance Z_(line)decreases. FIGS. 3 and 4 show the variations in the complex impedanceZ_(line) as the frequency increases. As shown, compensating for thesevariations in impedance using only resistive elements is virtuallyimpossible.

[0027]FIG. 5 illustrates a circuit having many elements that are thesame as elements of the circuit of FIG. 1 and are referred to with thesame reference numbers. Cancellation circuit 550 is coupled betweennodes T, −T, R and −R and the receiver 110 input nodes A and B.Cancellation circuit 550 includes four separate impedance branches 554a, 554 b, 558 a and 558 b that are coupled, respectively, between nodesT and A, and nodes −R and A, nodes −T and B, and nodes −R and B.

[0028] Impedance branch 554 a includes resistor R554 a 1 and capacitorC554 a 1 coupled in series. Impedance branch 554 b includes resistorR554 b 1 coupled in parallel with a series combination of resistor R554b 2 and capacitor C554 b 1. Impedance branch 558 a includes a seriescombination of resistor R558 a 1 and capacitor C558 a 1. Impedancebranch 558 b includes resistor R558 b 1 coupled in parallel with aseries combination of resistor R558 b 2 and capacitor C558 b 1. In oneimplementation, each of R554 a 1 and R558 a 1 has a value of 4.6 kΩ;each of C554 a 1 and C558 a 1 has a value of 16 nanoFarads; each of R554b 1 and R558 b 1 has a value of 1.7 kΩ; each of R554 b 2 and R558 b 2has a value of 7.1 kΩ; and each of C554 b 1 and C558 b 1 has a value of1 nanoFarad.

[0029] Each of the four impedance branches 554 a, 554 b, 558 a and 558 bincludes resistive elements (i.e., the resistors) and reactive elements(i.e., the capacitors). The use of both resistors and capacitorsproduces complex impedances. In other words, each branch has realimpedance components based substantially on the values of the resistorsand imaginary impedance components based substantially on the values ofthe capacitors.

[0030] The circuits shown in FIGS. 6-8 are analyzed to describe thebehavior of the circuit shown in FIG. 5. For brevity and clarity, onlyhalf of cancellation circuit 550 that includes impedance branches 554 aand 554 b is described. It should be understood that the followinganalysis also applies to impedance branch 558 a and 558 b of thecancellation circuit. Using superposition, several of the nodes, T, Rand −R are grounded and the resulting characteristic equations arecalculated. Also for the sake of brevity, the impedance of branch 554 ais defined as Z₁ and the impedance of branch 554 b is defined as Z₂.

[0031] The voltage at node A, V_(A), includes a component, V_(AT),attributable to the voltage at node T, and a component, V_(A−R),attributable to the voltage at node −R:

V _(A) =V _(AT) +V _(A−R).

[0032] In FIG. 6, node −R is grounded so that V_(A−R) equals zero andV_(AT) is calculated to determine the effect of echoing the transmittedvoltage onto nodes A and B. By voltage division, V_(AT) is:$V_{AT} = {{V_{T}\left\lbrack \frac{Z_{2}}{Z_{1} + Z_{2}} \right\rbrack}.}$

[0033] In FIG. 7, node T is grounded so that VAT equals zero and V_(A−R)is calculated to determine the effect of the voltage at node −R on nodeA. By voltage division, V_(A−R) is:$V_{A - R} = {{V_{- R}\left\lbrack \frac{Z_{1}}{Z_{1} + Z_{2}} \right\rbrack}.}$

[0034] As described earlier, V_(−R) is itself a combination of thesignals received through transformer 120 from outside circuits as wellas the signals output by transmitter 105 that are propagated to node −Rthrough terminating resistor 115 b. The voltage applied to node −R fromtransformer 120 due to received input signals is ignored.

[0035] Grounding node R in FIG. 5 produces the equivalent circuit shownin FIG. 8. The relationship between V_(−T) and V_(−R) is derived throughvoltage division to be:${V_{- R} = {V_{- T}\left\lbrack \frac{Z_{line}}{Z_{line} + R_{T}} \right\rbrack}},$

[0036] and substituting V_(T) for V_(−T) produces:$V_{- R} = {- {{V_{T}\left\lbrack \frac{Z_{line}}{Z_{line} + R_{T}} \right\rbrack}.}}$

[0037] Substituting for V_(−R), V_(AT), and V_(A−R) in the equation forV_(A) using the equations above yields:${V_{A} = {{V_{T}\left\lbrack \frac{Z_{2}}{Z_{1} + Z_{2}} \right\rbrack} + {{\left( {- V_{T}} \right)\left\lbrack \frac{Z_{line}}{Z_{line} + R_{T}} \right\rbrack}\left\lbrack \frac{Z_{1}}{Z_{1} + Z_{2}} \right\rbrack}}},$

[0038] which may be rewritten as:$V_{A} = {{\left\lbrack \frac{V_{T}}{Z_{1} + Z_{2}} \right\rbrack \left\lbrack {Z_{2} - {Z_{1}\left( \frac{Z_{line}}{Z_{line} + R_{T}} \right)}} \right\rbrack}.}$

[0039] From the preceding equation, it is clear that the transmittedoutput voltage V_(T) can be eliminated from nodes A and B if${Z_{2} = \frac{Z_{1} \times Z_{line}}{Z_{line} + R_{T}}},$

[0040] which may be rewritten as:$\frac{Z_{2}}{Z_{1}} = {\frac{Z_{line}}{Z_{line} + R_{T}}.}$

[0041] Thus, by designing the impedances within each of the branches incancellation circuit 550 to be correlate with the impedances of Z_(line)and the terminating resistors R_(T), the echo of the outgoingtransmission signal, V_(T) and V_(−T), into receiver circuit 110 isreduced or eliminated. In other words, as long as Z₂ varies in the sameproportion with Z_(line) as Z₁ varies in proportion with Z_(line) andR_(T), the reflection or echo of the transmitted output voltage into thereceiver 110 is reduced or eliminated. Impedance branches 554 a, 554 b,558 a and 558 b have complex impedances in order to correlate moreclosely with the complex impedance of the combination of Z_(line) andR_(T).

[0042] In FIG. 5, each impedance branch 554 a, 554 b, 558 a and 558 bincludes capacitors. Capacitors are reactive elements. By usingresistors and capacitors in the impedance branches, the frequencyresponse of echo cancellation circuit 550 more closely models thefrequency response of the transformer 120 and transmission lines 125 aand 125 b combination. Thus, the amount of the outgoing transmittedsignal from transmission circuit 105 that is echoed into receivercircuit 110 is attenuated or eliminated even as the impedance of thetransmission line and transformer combination varies with frequency.

[0043] Generally, cancellation circuit 550 operates as follows. As thefrequency of the output signals from transmitter 105 increases, theimpedance of transformer 120 increases. This results in more of theoutput transmission voltages V_(T) and V_(−T) being present on nodes Rand −R, respectively. To compensate for this, relatively largecapacitors C554 a 1 and C558 a 1 and resistors R554 a 1 and R558 a 1 areused to propagate more of the opposite polarity signals V_(T) and V_(−T)directly into nodes A and B, respectively. In other words, as thevoltage at node −R rises due to the increase in the impedance oftransformer 120, a larger portion of the opposite polarity signal V_(T)is propagated into node A through impedance branch 554 a to compensatefor the increase in voltage at node A caused by the increase in voltageat node−R. Thus, V_(T) is attenuated less by branch 554 so as to balancethe increase in the voltage at node −R. Similar behavior occurs at nodeB as a result of the behavior of impedance branch 558 a.

[0044] As the output signal frequencies increase beyond a certain point(e.g., 5 kHz), the impedance of transmission lines 125 a and 125 bincreases and the impedance of transformer 120 decreases. This causes anoverall decrease in Z_(line) as described above. With a decrease inZ_(line), the effect of the output voltage becomes less of a factor inV_(−R). However, V_(T) is still propagated to V_(A). To compensate forthis, V_(−R) is attenuated less so that a larger portion of V_(−R) isfed into node A. This is accomplished by having the impedance of seriescombination R554 b 2 and C554 b 1 decrease with increasing frequency.The decrease in impedance in that combination causes an overall decreasein impedance in branch 554 b and a resulting increase in the voltageV_(−R) propagated onto node A.

[0045] The echo cancellation circuit 550 has at least two advantagesover other echo cancellation circuits. First, no active elements areused. Thus, this circuit is relatively inexpensive and simple in designwhile still providing a close correlation to the combined impedance oftransformer 120 and transmission lines 125 a and 125 b. In addition, itdoes not need to be trained or biased with a DC power supply in order tooperate properly.

[0046] Second, the echo cancellation circuit 550 maps more closely withchanges in the combined transmission line and transformer impedanceresulting from changes in the frequency of the transmitted signals. Inother words, the echo cancellation circuit 550 better compensates forchanges in the transmission line and transformer impedance than echocancellation circuits that only include resistors. This is because eachbranch 554 a, 554 b, 558 a and 558 b has complex impedance (i.e., realand imaginary components). Thus, the outgoing transmission signalreflection into receiver 110 is substantially reduced over a wider rangeof frequencies.

[0047] A number of implementations have been described. Nevertheless, itwill be understood that various modifications may be made. For example,inductors can be used instead of the capacitors shown in FIG. 5. Whenusing inductors, the value and arrangement (i.e., serial vs. paralleland vice versa) with the resistors will differ from the arrangement andvalues of resistors described above. In addition, while oneimplementation has the resistors formed on an integrated circuit alongwith either the transmitter circuit 105, the receiver circuit 110, orboth, and the capacitors being discrete and external to the integratedcircuit, other implementations may have all elements of the cancellationcircuit integrated with the transmitter 105, the receiver 110, or both.Additionally, all elements of the cancellation circuit may beimplemented externally to the integrated circuit containing thetransmitter 105, the receiver 110, or both.

[0048] Accordingly, other implementations are within the scope of thefollowing claims.

What is claimed is:
 1. An echo cancellation circuit coupled between atransmitter and a receiver, the echo cancellation circuit comprising: afirst branch including real and imaginary impedances coupled between thetransmitter and the receiver; and a second branch including real andimaginary impedances coupled between the transmitter and the receiver.2. The echo cancellation circuit of claim 1 wherein the first branchincludes a resistor and a capacitor coupled together in series.
 3. Theecho cancellation circuit of claim 2 wherein the second branch includesa first resistor coupled in series with a capacitor and a secondresistor coupled in parallel with the first resistor and the capacitor.4. The echo cancellation circuit of claim 3 wherein the echocancellation circuit is implemented in a transmitter/receiver circuitwhich has a terminating resistor and a transformer coupled to atransmission line.
 5. The echo cancellation of claim 4 wherein a totalimpedance of the first branch is proportional to a combined impedance ofthe transformer and the transmission line combination over a range offrequencies.
 6. The echo cancellation circuit of claim 5 wherein a totalimpedance of the second branch is proportional to a sum of the combinedimpedance of the transformer and transmission line combination and animpedance of the terminating resistor over a range of frequencies. 7.The echo cancellation circuit of claim 6 wherein the range offrequencies is 0 kHz to 10 kHz.
 8. The echo cancellation circuit ofclaim 1 wherein the second branch includes a first resistor coupled inseries with a capacitor and a second resistor coupled in parallel withthe first resistor and the capacitor.
 9. An echo cancellation circuitcoupled between a transmitter and a receiver, the echo cancellationcircuit comprising: a first branch including real and imaginaryimpedances coupled between the transmitter and the receiver; a secondbranch including real and imaginary impedances coupled between thetransmitter and the receiver; a third branch including real andimaginary impedances coupled between the transmitter and the receiver;and a fourth branch including real and imaginary impedances coupledbetween the transmitter and the receiver.
 10. The echo cancellationcircuit of claim 9 wherein the first branch and the third branch eachinclude a resistor and a capacitor coupled together in series.
 11. Theecho cancellation circuit of claim 10 wherein the second branch and thefourth branch each include a first resistor coupled in series with acapacitor and a second resistor coupled in parallel with the firstresistor and the capacitor.
 12. The echo cancellation circuit of claim11 which is implemented in a transmitter/receiver circuit which has aterminating resistor and a transformer coupled to a transmission line.13. The echo cancellation of claim 12 wherein a total impedance of eachof the first branch and the third branch is proportional to a combinedimpedance of the transformer and transmission line combination.
 14. Theecho cancellation circuit of claim 13 wherein a total impedance of eachof the second branch and the fourth branch is proportional to a sum ofthe combined impedance of the transformer and transmission linecombination and the impedance of the terminating resistor.
 15. A methodof receiving an input signal and canceling an output transmission signalcomprising: transmitting the output transmission signal over a firstterminal and a second terminal; receiving the input signal over a thirdterminal and a forth terminal; attenuating a first part of thetransmitting signal through a first complex impedance; and attenuating asecond part of the transmitting signal through a second compleximpedance.
 16. The method of claim 15 further comprising: furthercomprising matching a line impedance through a first terminatingresistance and a second terminating resistance.
 17. The method of claim16 further wherein attenuating the first part of the transmitting signalcomprises dividing of a voltage of the first part of the transmittingsignal.
 18. The method of claim 17 wherein the dividing involves thefirst complex impedance being proportional to a combination of the lineimpedance and an impedance of the first terminating resistance lineimpedance.
 19. The method of claim 16 wherein attenuating the secondpart of the transmitting signal comprises dividing of a voltage of thesecond part of the transmitting signal.
 20. The method of claim 19wherein dividing involves the second complex impedance beingproportional to a line combination of the line impedance and animpedance of the second terminating resistance impedance.
 21. The methodof claim 15 further comprising amplifying the input signal.